1. Field of the Invention
The present invention relates to an integrated circuit (IC, hereinafter). More particularly, the present invention relates to a multiple-time programming apparatus and a method using one-time programming elements.
2. Description of Related Art
In a proceeding of an integrated circuit manufacture, the electrical parameters of each integrated circuit may vary from lot to lot, and vary from wafer to wafer of the same lot, and even vary from die to die of the same wafer. There are different process variations such as deviation of ion implantation, deviation of gate oxide thickness, and error in etching. Such variation of process will cause frequency deviation of an oscillator, or voltage deviation of a regulator. If the electrical parameters of an integrated circuit vary beyond the specification, e.g. over 5% deviation of the IC specification, the IC is identified as defective during testing process. Therefore, the IC manufacturers usually perform some post-fabrication fine-tuning of the parameters mentioned above in order to increase IC production yield. Since the aforementioned parameters always change along with the variation of process parameters, one-time programming (OTP, hereinafter) elements, e.g. fuse or metal wire, are used usually for fine-tuning to keep consistency of an output lot. Generally speaking, the methods of fine-tuning an IC at ex-factory effectively increase the production yield.
Common OTP adjusting methods for integrated circuit includes laser trim and poly fuse, also known as E-fuse. The OTP element used in laser trim is a metal wire, and it is programmed by blowing the metal wire with high energy laser. The OTP element used in poly fuse is a poly wire (a.k.a. poly fuse), and it is programmed by blowing the poly wire with a large current or by changing the resistance of the poly wire through the electron migration caused by a large current. To find out whether an OTP element is programmed, the metal wire or ploy wire is detected for open circuit or change of resistance by reading the circuit. The aforementioned programming process is irreversible, i.e. the element cannot be reprogrammed once it is programmed.
In using an OTP element, e.g. a poly fuse, the element cannot be programmed again once it is programmed (blowed), hence the parameter can not be re-adjusted, i.e. it is not multiple-time programmable. However, from the users prospective, it is desired that the parameters can be re-programmed or modified several times even after they are programmed by IC manufacturer at ex-factory. Take STN LCD driver ICs for example, although the operating voltage VLCD of STN LCD driving waveform is adjusted to an accurate value at ex-factory of the IC. However in the STN LCD module factory, the variation in characteristics of liquid crystal formula and deviation of cell gap may cause the contrast ratio deviation of the STN LCD module, which makes it a defective product. In such case, it is desired for the STN LCD module factory that the operating voltage VLCD of STN driver can be fine-tuned again to increase the production yield of STN LCD module.
Therefore, in the prior art, Multiple-Time Programming (MTP, hereinafter) elements, e.g. Erasable Programmable Read-Only-Memory (EPROM, hereinafter), Electrically Erasable Programmable Read-Only-Memory (EEPROM, hereinafter), and FLASH memory are adopted to perform multiple-time programming. However, MTP elements are unfavorable because of more expensive process, for example, a common STN LCD driver IC is manufactured with a 0.35 μm 3.3V/18V high voltage process. Adding a MTP element such as an EEPROM into the IC requires several additional masks, thereby increasing the manufacturing cost. The additional masks also reflect to longer manufacturing time and delivery lead-time, and lower production yield. Furthermore, fewer foundries are equipped with technologies for implementing the process fabricating MTP elements. Adopting MTP elements makes it difficult to find more appropriate subcontract foundries, hence endanger the productivity dispersion.